Electrets are dielectric materials with quasi-permanently embedded static electric charges and/or quasi-permanently polarized dipoles. Notably, electret materials are utilized in many commercial and technical applications, such as, for example, static electric sensing applications (e.g., electret microphones, copy machines), signal transmission applications (e.g., ULF/VLF transmitters operating at 30 kHz and below), and energy harvesting applications (e.g., deriving energy from external sources such as ambient vibrations, wind, heat or light).
The performance of an electret material is proportional to the material's charge density per unit volume of space, or C/m3. However, since the static electric charges often can only be placed on the surfaces of the electret materials involved, the charge densities of these electret materials are often expressed in terms of unit area of space or C/m2. Notably, existing electret materials having higher charge densities are utilized in certain applications to maximize the performance of the application devices involved. However, the maximum charge densities of the electrets are limited primarily by the electrical breakdown threshold level between the dielectric material of the electrets and the surrounding air.
Conventional electret research and development has been limited primarily to two-dimensional or flat-surfaced materials having maximum charge densities of approximately 30 mC/m2. However, the charge densities of these two-dimensional surfaces are considered relatively low due to the shallow penetration depths of the ionized charges. Notably, in many commercial and defense applications, electronic or electromechanical devices utilizing electret materials having much higher charge densities than 30 mCm2 are required. For example, in VLF signal transmission applications, which are utilized for critical long distance and underwater communications, VLF transmitting devices utilizing electret materials having equivalent charge densities of >1 C/m2 are required. In this regard, ULF/VLF transmissions are particularly useful for applications in which signal penetration through certain conductive media (e.g., water, metal, rock, building materials, and the like) is desired. Also, ULF/VLF transmissions are very useful for long range communications applications, because signals in these frequency ranges can be coupled to the virtual ionosphere-to-ground “waveguide” that surrounds the Earth and propagates such signals around the globe with very little attenuation.
Electret materials are generally separated into two groups: organic electret materials (e.g., polymer); and inorganic electret materials (e.g., silicon dioxide). Polymer electret materials generally have a low charge density (<5 mC/m2). Also, polymer materials are not compatible with conventional microelectromechanical system (MEMS) fabrication processes, and thus can present daunting design challenges, for example, in the scaling of arrays.
A notable advantage of silicon dioxide electret materials is that they are compatible with existing silicon MEMS fabrication processes and typically have a much higher charge density (e.g., 34 mC/m2) than organic electret materials such as polymers. Also, another advantage of silicon dioxide electret materials over polymer electret materials is that silicon dioxide electret materials can be embedded with either unipolar or dipolar charges.
For the reasons stated above and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the specification, there is a need in the art for very high (e.g., ultra-high) charge density electrets.